Calibration of Lyophilized Standards for Ristocetin Cofactor Activity of von Willebrand Factor (vWF) Requires vWF-Deficient Plasma as Diluent for Dose-Response Curves

Summary

Calibration of local standards for ristocetin cofactor activity of von Willebrand’s factor (vWF : Rcof) against reference preparations is required to achieve a better standardization of this measurement. We have observed that in calibration assays (multiple line parallel bioassay) for vWF : Rcof, measured aggregometrically, the type of diluent used to obtain the dose-response curves exerted a critical influence on the final potency estimation when a fresh-frozen plasma against a lyophilized standard or two lyophilized preparations were to be compared. This pattern was observed with various commercial standards and also with the First International Reference Preparation for Factor VIII-Related Activités in Plasma. The choice of diluent was not relevant if two fresh-frozen plasma preparations were to be calibrated. Systematic experiments using as diluents Trissaline, Tris-buffered-albumin solution or vWF-deficient plasma (vW-plasma) showed that the ratio lyophilized/fresh-frozen preparations was invariably lower with albumin solution than with vW-plasma. Tris-saline partially corrected this discrepancy, but yielded less precise estimates. Preliminary results with cryosuper-natant-plasma seems to indicate that this material is a valid substitute for vW-plasma, but confirmatory experiments are required For the moment, plasma obtained from patients with severe type III vW-disease should be considered the “ideal” diluent for calibration assays involving lyophilized preparations.

Keywords

von Willebrand factor - Ristocetin cofactor assay - Standardization - Lynphilized standards - von Willcbrand’s disease

• References

• 1 Miller CH, Graham JB, Goldin LR, Elston RC. Genetic of classic von Willebrand’s disease: II. Optimal assignment of the heterozygous genotype (diagnosis) by discriminant analysis. Blood 1979; 54: 137-145
• 2 Italian Working Group. Spectrum of von Willebrand’s disease: a study of 100 cases. Br J Haematol. 1977 35. 101-112
• 3 Barrowcliffe TW, Tydeman MS, Kirkwood T BL, Thomas DP. Standardization of factor VIII - Establishment of a stable reference plasma for factor VUI-related activities. Thromb Haemostas 1983; 50: 690-696
• 4 World Health Organization Technical Reports Series. 1983 687. 23-24
• 5 Rodeghiero F, Castaman G, Dini E. Epidemiological investigation of the prevalence of von Willebrand’s disease. Blood 1987; 69: 454-459
• 6 Macfarlane DE, Stibbe J, Kirby EP, Zucker MB, Gant RA, McPherson J. A method for assaying von Willebrand factor (ristocetin cofactor). Thromb Diathes Haemorrh 1975; 34: 306-308
• 7 Kirkwood T BL, Snape TJ. Biometric principles in clotting and clot lysis assays. Clin Lab Haematol 1980; 2: 155-167
• 8 Curtis AD. The statistical evaluation of factor VIII clotting assays. Scand J Haematol 1984; suppl 41 55-68
• 9 Howard MA, Firkin BG. Ristocetin - a new tool in the investigation of platelet aggregation. Thromb Diathes Haemorrh 1971; 26: 362-369
• 10 Baugh RF, Brown JE, Hougie C. Plasma components which interfere with ristocetin-induced platelet aggregation. Thromb Diathes Haemorrh 1975; 33: 540-546
• 11 Ts’ao CH, Green D, Rossi EC. Some factors affecting fibrinogen precipitation bv ristocetin: ultrastructure of precipitates. Blood 1975; 45: 621-629
• 12 Stibbe J, Kirby EP. The influence of Haemaccel, fibrinogen and albumin on ristocetin-induced platelet aggregation. Relevance to the quantitative measurement of the ristocetin cofactor. Thromb Res 1976; 8: 151-165
• 13 Zuzel M, Nilsson IM, Aberg M. A method for measuring plasma ristocetin cofactor activity. Thromb Res 1978; 12: 745-754
• 14 Weiss HJ, Rogers J, Brand H. Defective ristocetin-induced platelet aggregation in von Willebrand’s disease and its correction by factor VIlI. J Clin Invest 1973; 52: 2697-2707